A sealing machine typically has two seal tools at lest one of which is heated. In a standard application a lower thermoplastic film formed with an array of blisters each holding a respective small object is advanced along with a planar upper thermoplastic film between a pair of vertically spaced such seal tools. The lower plate has a planar upper face formed with an array of pockets corresponding to the blisters of the lower film, which is oriented so that the blisters are aligned with and can drop without contact into the pockets when the plates are pressed together. The upper plate has a lower face that can be a smooth continuous plane or also formed with an array of recesses so that it at least engages pinches the two films together with the lower plate in webs extending around the blisters, so as to hermetically close the blisters and encapsulate the objects therein. One standard application of this system is the manufacture of blister packs of pills. Alternately the two plates can be used simply to form the blisters in a single film.
As described in US 2006/0081609 of J. Bentele, the seal tool is formed of a three-layer laminate. The thick core layer is made of aluminum and is provided with heating passages through which a hot liquid is circulated or that are fitted with rod heaters. To each side is a hard steel plate, with an array of bolts extending between the outer plates to lock the three plates together into a rigid assembly. One of the outer steel plates serves as the seal face of the assembly. The other plate is provided to prevent differential thermal expansion of the aluminum and steel from deforming the seal tool.
Such an assembly is fairly expensive and, even when meticulously made, can deform somewhat. Any nonplanarity can result in regions of the two films being sealed that are either burnt through or not sealed together at all. Either way the result is a package reject that is unacceptable, especially when the system is used for an expensive medicament.